Insulating material



sistant to flaking.

Patented Jan. 1943 UNITED STATES PATENT OFFICE INSULATING MATERIAL Lawrence R. Hill, Edgewood, and Arthur L.

Brown, Pittsburgh, Pa., assignors to Westinghouse Electric & -Manufacturing Company,. East Pittsburgh, Pa.', a corporation of Pennsylvania No Drawing. Application May 6, 1939, Serial No. 272,138

5 Claims. (Cl. 166-237) bonded into sheet form, but these have not always been satisfactory where the insulating sheet is machine built, because the resulting sheet or plate lacks uniformity, mechanical strength and is diflicult to handle. 7

An object of this inventionis to provide an insulating material formed from mica flakes.

Another objectof this invention is to provide an insulating material formed from mica flakes, and which is mechanically strong, can be milled without fracture and isresistant to flaking.

A further object of this invention is toprovide an insulating material of mica flakes bonded with less than 3% of solid adhesives which renders the material mechanically strong and re- Other objects of this invention will become ,apparent from the following description and the appended claims.

In the making 'ofsheet or plate insulating material, mica flakes are desirable becausejof their high dielectric strength. In making the sheet of insulating material embodying this invention, mica flakes of a size commercially available are substantially evenly distributed over a travelling web in any of the known methodsto give a, predetermined thickness depending upon the desired thickness of the finished sheet or late.

In order to bond the nica flakes in their distributed position, an adhesive is slowly dripped on to the distributed 'flakes, after. which they are subjected to suflicient heat to drive ofi the solvent and give a sheet material which can be physically handled. In order to accomplish this result, a preferred bond comprising solid adhesives. shellac and gum elemi,"is employed in accordance with this invention.

In the bond, shellac and gum elemi form the body or solids thereof, while suitable solvents, such as alcohol and ethyl acetate, are employed to render the bond liquid for applying it to the distributed flakes. Shellac by itself is unsatisfactory for this purpose, since in practice it. is found that the sheets of the flakes bonded with the shellac from whichthe solvent is dried can- .hesion and which win. of itself effect an equali-' zation of the flow of the bond over and between the distributed mica flakes prior to subjecting it to the heat treatment, as describedherein- I after, gum elemi is employed in conjunction with the shellac The gum elemi forms from 2% to by weight of the solids in the'bond 'with the balance shellac. Since the gum alemi melts at about C., it is eiiective when employed in the proportionsgiven hereinbefore for causing the bond to flow for effecting the equalization there-' of over and between the mica flakes.

Since it is desirable to apply the bond to the mica flakes in a liquid form, it is necessary to provide a suitable solvent for the solids of the bond. Alcohol is found to be -a'satisfactory solvent for the shellac, but the gum elemi forms a colloidal suspension therein. In order to render the gum elemi liquid, it is added to ethyl acetate in whichit .is found tobe completely soluble. The solutions of shellac and alcohol and gum elemi in ethyl acetate readily mix when added together giving the desired liquid bond.

In the liquid form, the bond'comprises .not

over 10% by weight of shellac and, gum elemi with the balance solvents with a preferred solid constituents of'the bond ranges from to of alcohol andfrom 50% to5% of ethyl acetate depending upon the gum elemi content in the bond. Ifless than 2% of the gum elemi is employed, the desired equalization of the flow of bond and desired adhesion isnot obtained. If more than 30% of the gum elemi is employed in theliquid bond, it is found'that the resulting not be Physically handled during the processing mica flake sheet or plate lacks the required me-' chanical strength. In. practice, it isfound that about 18% of gum elemi in the bond gives the" desired results in the insulating sheet'or plate.

- After the bond is formedfrom the solids and I solvents in the preferred proportions described I -hereinbefore, it isdripped upon the distributed mica flakes in an amount to give not over 3% of bonding solids in the resulting sheet when it is heat treated as hereinafter described. The bond coated strip of mica flakes is carried into a furnace and subjected to a. temperature of about C. for a period of time of about 10 minutes to effect the-removal'oi the solvent therefrom and because of the inclusion of 811m elemi in the bond to cause an equalization of the flow of the. bondthroughout the mica flakes. As

the sheet or strip of mica flakes carrying the bond emerges from the furnace, it has sufiicient mechanical strength whereby it can be cut into sheets of predetermined size and be. physically handled for stacking.

After the sheets of mica flakes carrying the bond have been heated to drive off the solvent, they are stacked in suitable forms and placed in suitable presses to which heat at a temperature of about 170 C. is applied. The sheet carrying the bond is subjected to this temperature for a predetermined length of time to effect the polymerization of the shellac and gum elemi and to drive off the volatile fraction of the gum elemi. After the treatment at this temperature, it is found that the sheet is substantially solid and resistant to flaking.

'In making the sheet of insulating material embodying this invention, the preferred bond disthe speed of making the insulating sheets on automatic machines from 50% to 100%, thereby effecting economies in its manufacture. Further, the characteristics of the resulting sheet are independent of the speed of the web carrying the mica. flakes having the bond applied thereto and are independent of the temperature of the drying ovens so long as the temperature is sufliciently high to remove the solvent therefrom. The sheet or plate resulting after the first pressing during which the solids of the band are polymerized may be milled without danger of mechanical fracture and is stronger mechanically than any known mica sheet having a similar bond content. Further, in use, the gum elemi facilitates the burning off of the bond at temperatures of 220 C. or higher without leaving a carbon residue.

Although this invention has been described with reference to a particular embodiment thereof, it is, of course, not to be limited thereto except insofar as is necessitated by the prior art and the scope of the appended claims.

We claim as our invention:

1. As an insulating material, mica flakes bonded together with a mixture of shellac and gum elemi, the bond having been formed under'heat and pressure from aliquid solution comprising not over 10% by weight of combined shellac and gum elemi with the balance a solvent consisting of from 50% to 95% by weight of alcohol and from 50% to 5% by weight of ethyl acetate, the gum elemi melting at about 35 C. and constituting from 2% to of the mixture of shellac and gum elemi.

2. As an insulating material, mica flakes bonded together with less than 3% of a bond comprising shellac and gum elemi, the bond having been formed under'heat and pressure from a liquid solution comprising not over 10% by weight of combined shellac and gum elemi with the balance a solvent consisting of from 50% to 95% by weight of alcohol and from 50% to 5% by weight of ethyl acetate, the gum elemi melting at about C. andconstituting from 2% to 30% of the mixture of shellac and gum elemi.

3.A composition of matter comprising mica flakes and a resinous bonding solution comprising not over 10% by weight of a mixture of shellac and gum elemi with the balance a solvent consisting of from 50% to 95% by weight of alcohol and from 50% to 5% by weight of ethyl acetate,

the gum elemi melting at about 35 C. and concomprising not over 10% by Weight of shellac and gum elemi having melting characteristics at about 35 C. with the gum elemi ranging from 2% to 30% and the balance of the solution being a suitable solvent for the shellac and gum elemi, drying the mica flakes carrying the solution to drive ofi the solvent and effect an equalization of the flow of the resins of the bond throughout the mica flakes, and subjecting the dried mica flakes carrying the resins of the bond to sufficient heat and pressure to effect the polymerization of the I shellac and gum elemi and compact the bonded flakes into a substantially solid body.

5. The process of making an insulating inaterial comprising, applying a bond in the form of a liquid solution to mica flakes arranged in a predetermined manner, the liquid solution comsubjecting the dried mica flakes carrying the;

shellac and gum elemi to suflicient heat and pressure to effect the polymerization of the shellac and gum elemi and form the bonded flakes into a substantially solid body.

LAWRENCE R. HILL. ARTH UR L. BROWN. 

